Lettuce, Lactuca sativa L., is a commercially important fresh leaf crop belonging to the Cichoreae tribe of the aster family (Asteraceae) which includes such other important crops as sunflowers and artichokes. Lettuce is widely grown throughout the temperate and subtropical regions of the world and is used predominantly as a fresh green in the human diet.
There are six morphological types of lettuce: iceberg (crisphead), romaine (cos), butterhead, leaf, stem and Latin. Ryder, E. J., 1979, Leafy Salad Vegetables, AVI Publishing Company. These basic lettuce types frequently form the basis for grouping lettuces as is commonly seen in supermarkets, grocery and produce stores.
The crisphead type is the most common in the United States, while butterheads and romaines are the popular types in northern and southern Europe. Id. In the United States, California is the leading producer of lettuce. California produces crisphead, leaf and romaine lettuce. In 1995, California's cash receipts for crisphead lettuce amounted to about $987 million dollars. 1996 California Agricultural Resource Directory. Furthermore, also in 1995, California exported about $154 million dollars of crisphead lettuce to other countries such as Japan, Canada, the European Union and Korea. Id.
Terms used herein to describe plants are explained in “Guidelines for the Conduct of Tests for Distinctness, Homogeneity and Stability” UPOV Ref. No. TG/13/7, 16 Oct. 1993, which is hereby incorporated by reference.
The iceberg group lettuces are characterized by their relatively large, spherical, dense heads averaging 20.0 cm in diameter and 1000 g in weight, which are borne on a set of frame leaves that form the base of the plant. The heads are composed of leaves that are spirally arranged on a stem with greatly foreshortened internodes, are tightly clasping upon one another forming a sphere of broadly shaped (obovate) leaves, where the length and width of each leaf is nearly identical, having length to width ratio (L/W ratio) of 1.0 approximates 20.0 cm in length to 20.0 in width. The length to width ratio for commercial iceberg lettuce varieties ranges from about 0.5 to 1.0. Outer leaves range in color intensity from dark green (Royal Horticultural Society Color Chart 146A) to green (RHS 146B) with inner leaves ranging from very pale green (RHS 145C) to white or blanched (RHS 145D). Iceberg lettuces have a closed head formation. Iceberg leaves have a high content of water—hence the name, “crisphead.” More information regarding the general characteristics of iceberg lettuce may be found in Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company, which is hereby incorporated by reference.
The romaine group of lettuces are characterized by large, cylindrical, semi-firm heads averaging 30.0 cm in diameter and 800 g in weight, which are borne on a set of frame leaves that form the base of the plant. The heads are composed of leaves that are spirally arranged on a stem with greatly foreshortened internodes, are loosely clasping upon one another forming a roll of elongated, spatula-shaped (spatulate) leaves, where the length is normally 50% longer than the width, having a range of length to width ratios of 1.2 to 2.5, where 1.5 is most common. Romaine lettuces generally have a semi-open head formation. The name “romaine” comes from the French for “Roman”. Outer leaves range in color intensity from dark green (RHS 146A) to mid-green (RHS 146B) to light green (RHS 146C) with inner leaves ranging from green (RHS 146B) to light green (RHS 146D). More information regarding the general characteristics of romaine lettuce may be found in Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company.
A major disadvantage of existing iceberg varieties lies in their shape. While they are widely used in salads and sandwiches for their unique taste and crunchy texture, the iceberg's round shape makes it difficult to process, resulting in inefficient handling and waste. For example, the iceberg lettuce head cannot be used when separating individual leaves to allow for cleaning and the production of individual lettuce leaves. The deeply cup-shaped leaves make thorough cleaning difficult and this same shape prevents their use as individual leaves in sandwiches because of their inability to lie flat and thus they are broken and damaged. In contrast, such disadvantages are not present in the shape of romaine lettuces.
Another disadvantage of existing iceberg varieties lies in their growing pattern. Because they grow low to the ground, harvesting must be done by hand. This means high harvest costs due to hand labor, which can also cause potential damage to the head when the lettuce stem is not cut at the correct level. Improperly harvested heads cannot be salvaged and must be discarded. In contrast, romaine lettuces grow erect and their leaves are oriented nearly vertically, thereby allowing for better access to the stem. These erect plants (such as a romaine) make the prospect of machine harvesting possible. Machine harvesting results in significant time and money savings.
Another disadvantage of existing iceberg varieties lies in their susceptibility to certain debilitating diseases. All plants rely on evaporation of moisture from the surface of their leaves to draw vital water and nutrients up into the remote areas where growth occurs. Due to the round head shape for crisphead lettuce, inner leaves are not exposed to light and air, thereby inhibiting evaporation of moisture from its inner leaves and the translocation of critically needed water, nutrients, and defensive agents to all interior areas of the head. Under these circumstances, the lack of calcium and other minerals in the process of leaf formation is a common problem in iceberg lettuce, causing brown and black spots (tipburn) to occur leading to the likelihood of subsequent infection by secondary pathogens such as bacteria and fungi, that result in the rapid decay and spoilage of the head from the inside out. In contrast, such diseases are much rarer in romaine lettuces due to the erect, semi-open shape of the head.
Other problems with existing cultivars adapted to western conditions include a lack of resistance to corky root rot and lettuce mosaic virus. Corky root rot is believed to be caused by a pathogenic soil bacterium of the genus Rhizomonas. One species of Rhizomonas that is commonly found to cause corky root rot is R. suberifaciens. Corky root rot accounts for significant lettuce crop loss in the western United States, particularly in the valleys of the central coast of California, i.e., the Salinas, Santa Maria, and Lompoc valleys.
Lettuce mosaic virus, on the other hand, is commonly found throughout the world, and occurs in all lettuce production areas of the United States. Vectored by the Green Peach aphid (Myzus persicae), outbreaks of lettuce mosaic virus can devastate an entire field within a short period of time.
Corky root rot symptoms include yellow bands on tap and lateral roots of lettuce seedlings. Guide to Leafy Vegetable Production in the Far West, Ron Smith, ed., California-Arizona Farm Press (1997). Yellow areas gradually expand and develop a green-brown color with cracks and rough areas on the root surface. The entire taproot may become brown, severely cracked and may cease to function. Feeder root systems are reduced and damaged. Roots become very brittle and break off easily. When the root is severely discolored, aboveground symptoms show up as wilting during warm temperatures, stunting and general poor, uneven growth. Loss of the root system results in stunted plants that are chlorotic and too small to harvest.
Lettuce mosaic virus symptoms first appear as vein clearing in the newly developed leaves followed by mottling (a mosaic appearance) and recurving of the leaves as they mature. Ryder, E. J., Leafy Salad Vegetables, AVI Publishing Company. The leaf margins increase in undulation and necrotic spots may also appear. Infected plants are generally much smaller than healthy plants, rendering the plants unsuitable for harvest.